Add support for VNC specific passwords

[jyundt]

Any thoughts on adding support for a separate VNC specific password?

I ran into an issue where my AMT password did not meet the RFBPassword / VNC password requirements because it was > 8 characters.

Maybe prompt the user when enabling KVM/VNC and update hosts.cfg?

Let me know if this sounds reasonable and I'll submit a PR to add this functionality.

[sdague]

A separate password for VNC would be great.

How about add a flag to the enable vnc sub command that will trigger the prompt, so it's optional.

[jyundt]

A separate password for VNC would be great.

Cool, I'll take a stab at this.

How about add a flag to the enable vnc sub command that will trigger the prompt, so it's optional.

Parsing an additional flag for a possible VNC password might get a little bit awkward because the "op" (on|off|reboot|pxeboot|status|vnc|vncstatus) is not a sub-command.

Do you mind if I get a separate PR going to modify the argparse config to include sub-commands?

Also, thanks for writing amt! I just got a vPRO enabled NUC and this utility is a life saver.